This bug not allow you to set own resolution of tiles, and allow to only use values provided by program. So if you want to set tile to 500 - WM will drop settings to nearest one - to 256 - it is bad, you know
Step to reproduce:
Bug is only works, if your check box “Always export all tiles” is active. While it active you not allowed to use 500, 1000, and all other non-regular values
If you will un-check this box - you will obtain ability to set custom ones.
This is fixed in 4045, along with a variety of other tiled setting UI bugs (such as having the settings modify the active scene, not the selected scene in the outliner).